Oligonucleotides are known to be very important, useful substances in the field of biology and medical science: as probes that recognize and detect specific sequences in the detection of nucleic acids; as primers for the use in PCR method which is an essential technique in genetic engineering; and further as antisense oligonucleotides for the use in the antisense method which is actively researched in recent years in the field of gene therapy.
In order for the aforementioned antisense oligonucleotides to manifest their respective functions, they all need the ability of their base sequence moiety to hybridize with complementary base sequences--what is known as "hybridization ability".
Further, it is required that the aforementioned antisense oligonucleotides possess not only the hybridization ability, but also the stability in vivo. For example, some attempts have been made concerning the control of genetic information by the use of antisense oligonucleotides. See, Zamecnick, Stephenson et al., Proc. Natl. Acad. Sci., U.S.A., 75, 280-284 (1978). The oligonucleotides which are used for this purpose are normally derived from nature and many of them have only, extremely low resistance to nuclease. This presents a problem that they are susceptible to undesired decomposition reactions in vivo. Consequently, a variety of such modified oligonucleotides that remedy these drawbacks are actually being developed.
One such example is an oligonucleotide with a phosphorothioate bond, which is referred to as "S-Oligo" (DeClercq et al., Science, 165, 1137-1139 (1969)), and it can be easily synthesized on a DNA auto-synthesizer. It is known that this kind of oligonucleotides have substantial resistance to nuclease. See, Wickstrom et al., J. Biol. Biophys. Meth., 13, 97-102 (1986). Another example is an oligonucleotide with a methylphosphonate bond, which is referred to as "MP-Oligo" (Miller et al., Biochemistry, 18, 5134-5142 (1979)). Substitution of one of the oxygen atoms present in phosphoric ester bonds of DNA of the natural type with a methyl group provides the nucleotide with resistance to nuclease, and in addition, it eliminates a charge at the phosphoric acid moiety, thus substantially improving membrane permeability.
However, the aforementioned S-Oligo is a racemic mixture including many isomers with their chiral centers at phosphoric ester bond moieties, and has a drawback in that it is provided with a low affinity to RNA or DNA. A further drawback is that it does not possess sufficient stability in vivo (i.e., resistance to nuclease).
Also, the MP-Oligo is a racemic mixture like the S-Oligo, and has a drawback in that it is provided with a low affinity to RNA or DNA. A further drawback is that its water solubility is low because there is no charge at the phosphoric ester bond moieties.
Furthermore, the still further drawback of the antisense oligonucleotides known in the art is that once they have been introduced in vivo, they can not be controlled with regard to the expression of their activities such as concentrations, sites, and time.